Analog-to-digital converters are commonly used in a variety of applications including computer-controlled manufacturing systems, communication systems and signaling systems. In these systems, the analog-to-digital converter samples voltages which are produced by transducers that measure analog phenomena occurring in the environment. The analog-to-digital converter converts the magnitude of the voltage into digital signals that can be conveniently stored and processed in conventional digital computers.
It is well-known that the rate at which samples of an analog signal are taken must be at least twice the maximum frequency component of the analog signal in order to insure that the signal can be reproduced accurately. As the operating frequency of electronic processing circuitry becomes higher and higher, it has been desirable to greatly increase the sampling and conversion speed of analog-to-digital converters to provide digital data at a rate that will not limit the processing rate of the digital processing circuitry.
Several different technologies have been used in the prior art to fabricate high-speed analog-to-digital converters. For example, one such prior art technology is silicon bipolar circuits. Converters using silicon bipolar technology generally consist of a track-and-hold circuit followed by a parallel bank of comparators. The outputs of the comparators are combined by logic circuits to generate the final digital output. Sampling rates of 400 million samples per second have been built utilizing this technology, but the distribution of analog signals to the comparators involves serious signal interference and capacitance loading problems. In addition, the large number of comparators which must be placed in a small area also presents heat dissipation problems that practically limit the resolution of the ultimate system to a small number of output bits.
Accordingly, other prior-art analog-to-digital converters have been made using gallium-arsenide (GaAs) technology. Due to the physical characteristics of gallium arsenide, GaAs circuits have an intrinsically higher speed than silicon bipolar circuits. Accordingly, in theory, the conversion speed can be significantly improved utilizing this alternate technology. However, at present, GaAs circuits are limited in performance due to unstable transistor operating characteristics.
Other prior art converters have utilized Josephson-junction superconducting quantum interference devices (SQUIDs). The switching characteristics in these devices are a periodic function of the magnitude of the input analog signal and thus the devices can be used as a basic element of an analog-to-digital converter. In such a converter, the analog signal to be converted is applied simultaneously to an array of SQUID devices connected in parallel.
SQUID devices have the advantage that they are small and capable of operating at fairly high speeds. Unfortunately, due their operating characteristics, SQUID switching thresholds are established by the peak amplitude of the clock pulses which drive the devices. Thus, during converter operation, the clock pulse must be held constant from cycle to cycle to prevent quantization error. Stable clocks are difficult to design at the speeds required.
In addition, SQUID devices require that the analog signal not vary during the conversion process to prevent erroneous switching. Accordingly, the converter must incorporate fast, accurate sampling or be preceded by very fast track-and-hold circuit. Both of the latter functions are difficult to design at the high operational speeds necessary.
Parallel SQUID converters also encounter problems with digital crosstalk into the analog system and instability of quantization thresholds. The first problem requires careful design of chip mounting and the second problem requires that the converter be equipped with automatic recalibration systems thereby increasing the cost of the system and its complexity.
Consequently, to increase conversion speeds, a variety of electrooptic techniques have been tried, including multi-interferometer electrooptic converters utilizing either Taylor or Mach-Zehnder interferometers. In constructing analog-to-digital converters, optical interferometers have been used as a modulator with a sampling input, an input for the analog signal to be converted and an output for the modulated signal. The analog input is applied to electrodes in the device which alter the magnitude of the signal at the output.
A common pulsed optical signal generated from a pulsed laser is applied in parallel to a plurality of interferometric modulators. Generally, a separate interferometric modulator is used for each bit of the digital output signal. The modulator outputs are sensed by photodiodes and compared to a reference signal obtained from the common laser source to generate a digital output signal.
A prior art electrooptical converter of this type using a Mach-Zehnder interferometer is described in detail in an article entitled "Wide Band Electrooptic Guided-Wave Analog-To-Digital Converters", by R. A. Becker, Charles E. Woodward, F. J. Leonberger, and Richard C. Williamson, Proceedings of the IEEE, Volume 72, No. 7, July 1984, which is hereby incorporated by reference.
While the above-described electrooptical converters operate satisfactorily and can sample at a one gigasample per second rate with two-bit and four-bit resolution, they suffer from two problems. One problem is that high-resolution systems with a large number of output bits require a large number of optical modulators connected in parallel. Each optical modulator must be connected in parallel to the analog voltage source and it has been found that the capacitance of all of the converter electrodes connected in parallel makes it extremely difficult to drive a large modulator array with conventional RF amplifiers at reasonable operating speeds. Thus, it appears that the parallel electrooptical converter arrangement is limited by practical RF amplifier considerations to low-resolution systems.
Accordingly, it is an object of the present invention to provide a simplified electrooptical analog-to-digital converter.
It is another object of the present invention to provide an electrooptic analog-to-digital converter which does not require an electrooptic interferometer for each bit of resolution.
It is still another object of the present invention to provide an electrooptic analog-to-digital converter which is capable of high-speed operation.
It is yet a further object of the present invention to provide an electrooptic analog-to-digital converter which does not suffer from high analog input capacitance due to multiple parallel electrodes.
It is still another object of the present invention to provide an electrooptic analog-to-digital converter which can be constructed using conventional interferometric technology.
It is a further object of the present invention to provide an electrooptical analog-to-digital converter which produces outputs that are compatible with conventional digital circuitry.